Early Pleistocene (Marine Isotope Stage 40–36) paleoceanography in the northwestern Pacific: Evidence from faunal and oxygen isotope analyses of planktonic foraminifera

The Mid-Pleistocene Transition (MPT) is characterized by a shift from a 41-kyr to a 100-kyr climate cycle of approximately 1.25–0.7 Ma. However, proxy records before the MPT are sparse in the northwestern Pacific region. To investigate the marine environment around the oceanographic convergence zone of the northwestern Pacific prior to the MPT, we conducted biostratigraphic and oxygen isotope analyses of planktonic foraminifera derived from the upper part of the Kiwada Formation of the Kazusa Group, which is distributed in the Boso Peninsula, central Honshu, Japan. The interval investigated was deposited from Marine Isotope Stages (MIS) 41 to 36 (ca. 1.31–1.20 Ma). The last occurrence of Globigerinoides obliquus is emplaced 30 m above the tephra layer Kd1 during MIS 37. The first occurrence of Neogloboquadrina inglei is placed somewhere below the tephra layer Kd8 and is older than 1.310 Ma. Time-series environmental proxy changes in the planktonic foraminiferal assemblages and oxygen isotopes suggest that the marine environment off the Boso Peninsula was influenced by the Kuroshio Current and the transitional water close to the Kuroshio Front, while the latitudinal shifts of the Kuroshio Front were associated with the glacial-interglacial periods prior to the MPT. Our results suggest that obliquity variability played a significant role in the mid-latitude Pacific Ocean before the MPT, and the hydrographic change in the North Pacific Subtropical Gyre seems to be connected to high-latitude climatic changes. Moreover, the temporary southward migration of the Kuroshio and Subarctic fronts occurred during the late MIS 38, which may be associated with the stronger East Asian winter monsoon. Thus, mid-latitude Pacific oceanographic changes that occurred before the MPT may be linked not only to global remote connections but also to local atmospheric circulation.